This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-211529, filed Jul. 12, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a developing apparatus in which magnetic toner is caused to adhere to the peripheral surface of a sleeve, utilizing magnetic force of a magnetic shaft, and an image forming apparatus, such as a laser printer or a copying machine, containing the developing apparatus.
An image forming apparatus, such as a laser printer, comprises: a photosensitive drum having a surface on which an electrostatic latent image is formed; a developing apparatus for developing the electrostatic latent image, thereby forming a toner image; and a transferring apparatus for transferring the toner image to a recording paper sheet.
Conventionally, a developing apparatus, in which magnetic toner is supplied to the photosensitive drum via a developing roller, is known as the developing apparatus for use in the aforementioned image forming apparatus. This type of developing apparatus comprises a toner container storing the magnetic toner, and the developing roller is supported by the toner container. The conventional developing roller comprises a hollow cylindrical sleeve made of non-magnetic material, and a magnetic shaft located inside the sleeve. The sleeve and the magnetic shaft are rotatable in synchronization with the photosensitive drum.
The sleeve is arranged parallel with the photosensitive drum. The sleeve has a peripheral surface for holding magnetic toner. The peripheral surface faces the surface of the photosensitive drum at a developing position defined between the sleeve and the photosensitive drum. The magnetic shaft has a plurality of south poles and north poles arranged alternately along the circumferential direction. The magnetic shaft is disposed inside the sleeve parallel thereto. The magnetic shaft and the sleeve are arranged coaxially such that their centers of rotation coincide with each other. Therefore, a gap, which is uniform along the circumferential direction, is formed between the sleeve and the magnetic shaft.
In the developing apparatus as described above, the magnetic toner stored in the toner container is attracted to the peripheral surface of the sleeve by the magnetic force of the magnetic shaft. The magnetic toner adhering to the peripheral surface is guided to the developing position in accordance with the rotation of the sleeve, and caused to adhere to the photosensitive drum at the developing position. As a result, the electrostatic latent image on the surface of the photosensitive drum is developed and a toner image is formed on the surface.
In the conventional developing apparatus, a biasing voltage is applied to the sleeve of the developing roller. For this reason, an electric field is generated at the developing position due to a difference between the potential of the electrostatic latent image on the photosensitive drum and the biasing voltage. A magnetic field generated by the magnetic shaft is also present at the developing position. The electric field applies force to cause the magnetic toner adhering to the peripheral surface of the sleeve to fly toward the electrostatic latent image on the photosensitive drum. To the contrary, the magnetic field applies force to keep the magnetic toner on the peripheral surface of the sleeve. At this time, since the magnetic shaft rotates within the sleeve, the south poles and the north poles alternately pass by the developing position. Therefore, an alternating magnetic field is generated at the developing position, with the result that the magnetic toner repeatedly moves between the electrostatic latent image on the photosensitive drum and the peripheral surface of the sleeve.
In the conventional developing apparatus, the radius of the magnetic shaft is smaller than that of the photosensitive drum. Accordingly, the curvature of the peripheral surface of the magnetic shaft is greater than that of the photosensitive drum. Since the conventional magnetic shaft is coaxial with the sleeve, after a point on the peripheral surface of the magnetic shaft passes by the developing position, it rapidly moves away from the surface of the photosensitive drum as the magnetic shaft rotates. Hence, at a position away from the developing position, the magnetic force applied to the electrostatic latent image on the surface of the photosensitive drum is very weak, resulting in slow movement of the magnetic toner between the electrostatic latent image and the sleeve.
Therefore, at a position away from the developing position, the magnetic toner, which should move between the electrostatic latent image on the photosensitive drum and the peripheral surface of the sleeve, floats therebetween. The floating magnetic toner is attracted to the surface of the photosensitive drum under the influence of the electric field.
As a result, the floating magnetic toner may adhere to the surface of the photosensitive drum like dots, at a position away from a region where an image is to be formed, more specifically, at a position deviated frontward from the developing position along the direction of rotation of the photosensitive drum. Since the magnetic toner dotted on the photosensitive drum is transferred to a recording paper sheet, the sheet will be stained with black.
The present invention was made in view of the above situation. Accordingly, an object of the present invention is to provide a developing apparatus and an image forming apparatus, in which magnetic toner is prevented from adhering to the surface of the photosensitive drum at a position deviated frontward from the developing position along the direction of rotation of the sleeve, so that a satisfactory image without a stain can be obtained. To achieve the above object, according to a first aspect of the present invention, there is provided a developing apparatus comprising a rotatable hollow cylindrical sleeve and a magnetic shaft rotatable inside the hollow cylindrical sleeve, wherein the hollow cylindrical sleeve has a peripheral surface for holding magnetic toner and the peripheral surface faces the surface of a cylindrical photosensitive body at a developing position, wherein a center of rotation of the magnetic shaft is deviated from a center of rotation of the sleeve nearer to the photosensitive body and frontward along a direction of rotation of the sleeve from a reference line connecting the center of rotation of the sleeve and a center of rotation of the photosensitive body, and wherein a line connecting the center of rotation of the sleeve and the center of rotation of the magnetic shaft crosses the reference line at a crossing angle "THgr"set within a range of 20xc2x0 less than "THgr"xe2x89xa660 xc2x0. With the above structure, the magnetic shaft is closest to the peripheral surface of the sleeve at a position deviated frontward from the developing position along the direction of rotation of the sleeve. Therefore, even after a point on the magnetic shaft passes by the developing position, the magnetic force of the magnetic shaft exerted on the photosensitive body is not reduced suddenly. As a result, the magnetic force is kept as strong as that at the developing position. Thus, at a position away from the developing position, the magnetic toner existing between the sleeve and the photosensitive body can be attracted to the peripheral surface of the sleeve. In addition, according to another aspect of the present invention, the magnetic shaft has a radius R3 (mm), the cylindrical photosensitive body has a radius R2 (mm), the magnetic shaft has magnetic force G (gauss), and the radius of the magnetic shaft and the radius of the photosensitive body have a relationship that satisfies G xc3x97(R3/R2) xe2x89xa7350. With this structure, since the curvature of the magnetic shaft is small, the radius of the magnetic shaft is almost the same as that of the photosensitive body. Therefore, even after a point on the magnetic shaft passes by the developing position as the magnetic shaft rotates, it does not rapidly move away from the surface of the photosensitive body. Hence, since the magnetic force of the magnetic shaft exerted on the surface of the photosensitive body does not reduce suddenly, the magnetic toner existing between the sleeve and the photosensitive body can be attracted to the peripheral surface of the sleeve. 